Its really depends on your resolution in order for it to make sense to get a 2gb card.
It almost never matters for cards in that performance category. Tests have shown the 1GB version of the HD 6950 performs just as well through all resolutions as the 2GB version. By the time it would matter cards like that have run out of GPU power.http://www.techpowerup.com/reviews/AMD/HD_6950_1_GB/22.html
O.P., those cards are neck and neck in performance. The best deal should be what drives your purchase.